Lack of food. He was quite thin when I got him, and although he's eating lots he's just nowhere near as active as the female - although maybe that's normal? She is forever whiring about whereas he tends to just hang around in the back of the tank on the ground, ocassionally whizzing up the back wall. Always on for a bit of nookie when the lights go out, so who knows? I'll just keep making food available and hope he gets a bit more confident and active.
Incidentally, I tipped a dozen or so small live mysids/mysis in last night. The female made a bee-line for them and chased one under a rock, then dug it out with her fin before chomping it! Quite suprising to see - she really did hunt it!